The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with accelerating data acquisition when using multiple radio frequency coils and will be described with particular reference thereto. It is to be appreciated, however, that the present invention may also find other applications, and is not limited to the aforementioned application.
In magnetic resonance imaging, a substantially uniform main magnetic field is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a patient being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the main magnetic field by transmitting radio frequency excitation signals into the examination region. Typically, radio frequency pulses transmitted via a whole body radio frequency coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding radio frequency magnetic resonance signal as it relaxes and returns to its former state of alignment with the main magnetic field. The resonance signals are received either by the whole body RF coil or by a localized coil, such as a head coil. Magnetic field gradients are applied during this process to encode spatial information in the phase and frequency of the resonance signal. Typically, read gradients cause frequency encoding of each data line along one axis and phase encode gradients step the data lines along an orthogonal data line. An image representation is reconstructed for display on a human viewable display.
In magnetic resonance imaging, relatively large amounts of data are collected when compared to other imaging modalities. For instance, in nuclear imaging, less actual data is typically collected, and large amounts of post-patient processing are performed on the data to generate a quality image. In MRI, large amounts of data are collected, but relatively little processing is performed to produce an image. Typically, a Fourier transform is applied to convert the k-space data from a frequency and phase space matrix into a real space image. Consequently, the patient is present in the scanner for periods of time which can be quite extended, depending on scan parameters, selected sequences, imaged volume, resolution, and other factors.
In an effort to shorten data collection times, a number of data collection sequences have been developed that are faster than spin-echo sequences. Some of these sequences use less than 90xc2x0 tip angles, in an effort to shorten the decay times of the excited spins. Others take different, non-raster paths through k-space. Several other time saving sequences have been developed to reduce scan times. As side effects of some of these sequences, signal to noise ratios are decreased, data collection is incomplete, or image artifacts are generated. Most sequences that speed the data collection process sacrifice some aspects of the final image.
The present invention contemplates a new and improved method and apparatus that speeds the data collection process without having to sacrifice image quality, overcoming the above referenced disadvantages and others.
In accordance with one aspect of the present invention, a magnetic resonance apparatus is provided. A main magnet assembly generates a substantially uniform main magnetic field through an imaging region. A gradient coil assembly superimposes gradient fields upon the main magnetic field. A first radio frequency assembly excites and manipulates dipoles in a subject disposed in the imaging region. The assembly receives magnetic resonance signals from the imaging region. The first radio frequency assembly has at least two modes, one mode being tuned to a sinusoid function along one axis to receive a different harmonic of the magnetic resonance signals than another mode of the assembly.
The coils of the radio frequency assembly may take on different geometries like, for example, the form of a planar ladder array or a cylindrical birdcage coil with a circular or elliptical cross section. A single physical coil may, in practice, be used in place of two or all three coils. For example, the imager""s body coil may be used in place of the first coil that transmits the radio frequency signals. This same body coil may also be used in place of the uniform receive coil. Alternatively, two separate resonance modes on a single coil structure could act as two of the coils in the above general discussion. For the purposes of this patent, however, we will describe the receive coils as if they are separate coils, knowing that they may not be separate physical coils in practice.
In accordance with another aspect of the present invention, a method of magnetic resonance is provided. A main magnetic field is induced through a region of interest in an imaging region of a magnetic resonance imaging apparatus. The main magnetic field is spatially encoded, and magnetic resonance is induced in selected dipoles in the imaging region. Magnetic resonance signals are received with a first coil structures, the coil having a first mode that receives signals that correspond to a first section of k-space. Magnetic resonance signals that correspond to a second mode corresponding to a second region of k-space are received concurrently. The received magnetic resonance signals are reconstructed into an image representation.
In accordance with another aspect of the present invention, a birdcage coil is provided. A ladder shaped coil array in planar form or shaped into the form of at least part of a cylinder having a region of sensitivity sensitive to an electromagnetic field that varies sinusoidally in a first dimension within the region of sensitivity.
One advantage of the present invention resides in faster data acquisition.
Another advantage of the present invention resides in shorter scan times.
Another advantage resides in increased patient throughput.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.